The present invention relates to a dielectric filter device having a plurality of juxtaposed coaxial dielectric resonators.
An example of a conventional dielectric filter device of this kind is disclosed in Japanese Patent Kokai No. 3-136502 in which the dielectric filter device comprises a plurality of juxtaposed coaxial dielectric resonators, each of the dielectric resonators includes an outer conductive layer provided on its outer surface except its front surface and an axially extended bore whose inner surface is coated with an inner conductive layer. Each dielectric resonator is covered with a printed-circuit board with which a plurality of conductors having a predetermined pattern are connected so as to form a capacitor circuit which generates a coupling capacitance. Each conductor is connected with the inner conductive layer of the associated coaxial resonator. Input and output terminals are connected with the inner conductive layers of the outermost positioned coaxial resonators with which capacitors may be externally connected to maintain input and output capacities. Also, in order to adjust a frequency response of the filter device and reduce the length of each resonator stray capacities may be provided. In that case a capacitor is connected between the inner conductive layer of each coaxial resonator and a ground terminal.
However, such a conventional filter device has disadvantages that the arrangement becomes complicated and is bulky because the input and output coupling capacities and the couping between the resonators are made by the same dielectric substrate and it is necessary to provide elements to be externally mounted other than the printed-circuit board for ensuring the required capacities and the stray capacities. Also it is difficult or substantially impossible to set and adjust the capacities after the filter is assembled.
Japanese Patent Kokai No. 61-156903 discloses another conventional filter device which comprises a plurality of juxtaposed coaxial dielectric resonators each including an axially extended bore whose inner surface is provided with an inner conductive layer, a connecting terminal having one end inserted into the bore and connected with the inner conductive layer and other end extended from the front surface, and an outer conductive layer provided on the intended portion of the outer surface. Each resonator is constructed by superimposing two dielectric block halves divided longitudinally in such a manner that bore halves provided in them are opposite so as to complete the bore.
With the arrangement disclosed in this reference, it is essential that lead wires should be connected with the inner conductive layers positioned at both the outermost coaxial dielectric resonators of the filter by soldering or the like. Therefore, lead wire guiding holes should be provided on both side portions of the dielectric block halves in a direction orthogonal to the axis of the bore halves. Each lead wire is guided outwardly through the corresponding hole. The guiding holes are given an enough inner dimension to prevent the thickness of the lead wires or any solder from obstructing the jointing of the dielectric block halves. However, to form the block halves with such lead wire guiding holes a complicated mold must be prepared, which results in that not only the forming of the block halves is costly, but also the block halves is weakened because of the present of lead wire guiding holes.